The present invention pertains to the electrical signal processing art and, more particularly, to a means for frequency stabilizing a signal generator.
Automatic frequency control systems are well known in the radio communications art. The function of such systems is to maintain a controllable oscillator at a selected frequency. Applications for such frequency stabilized signals include the injection oscillator of a receiver, or the carrier frequency generator in a transmitter.
A standard approach to automatic frequency control has been the use of two RF paths feeding a balanced detector. The first path couples the oscillator directly to the discriminator, while the second path passes the oscillator signal through a tuned circuit, tuned to the desired frequency of the oscillator. Since the oscillator signal coupled through the second path will be phase shifted as a function of its frequency relationship to the tuned circuit, the output from the balanced detector may be used as an error signal which, when fed back to the controllable oscillator, tends to maintain the oscillator at the selected frequency.
A serious disadvantage to the known automatic frequency control, or frequency discriminators, of the prior art is that a change in the DC characteristics of the balanced detector results in an error in the tuning of the controllable oscillator. Commonly, the balanced detector is comprised of a pair of detector diodes. Due to aging or thermal effects the diodes are known to have a varying rectification efficiency, whereby an output offset DC voltage can be expected.
Thus, the signal generating art has felt a need for a frequency stable oscillator whose stability is immune from DC drifts in its component elements, especially balanced detector drift.